D/A conversion circuit and semiconductor device

ABSTRACT

A D/A conversion circuit in accordance with the present invention, which is provided with a switch swD, allows a writing operation of a voltage (a true gradation voltage) to be performed at a higher speed by first applying a first voltage (a voltage close to the true gradation voltage), which is supplied without passing through a resistor element, to an output line and then applying a second voltage (the true gradation voltage), which is supplied via the resistor element, to the output line. Thus, the present invention can provide a D/A conversion circuit capable of writing display data to liquid crystal cells with higher precision at higher speed, and a semiconductor device utilizing such a D/A conversion circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a D/A conversion (digital analogconversion) circuit, and more particularly, to a D/A conversion circuitto be used in a driver circuit of a semiconductor device.

It should be noted that in this specification, the term “semiconductordevice” is intended to generally indicate all of such an apparatuscapable of operating by employing semiconductor properties. For example,the semiconductor device can include an electro-optical device, asemiconductor circuit, or an electronic equipment.

2. Description of the Related Art

In recent years, the technique for manufacturing a semiconductor devicee.g. a thin film transistor (TFT), in which a semiconductor thin film isformed on an inexpensive glass substrate shows rapid advancement becauseof increased demand for an active matrix type semiconductor displaydevice (in particular, an active matrix type liquid crystal displaydevice).

The active matrix type liquid crystal display device includes severalhundred thousands to several millions of pixel regions arranged inmatrix. Each of the pixel regions is provided with a TFT which controls,through its switching function, electric charges to be supplied to ordischarged from a corresponding pixel electrode.

Specifically, with high finer processing and higher image quality of adisplay device being required, an active matrix type liquid crystaldisplay device of digital driving type capable of operating at a higherspeed has drawn much attention.

FIG. 19 shows a conventional active matrix type liquid crystal displaydevice of digital driving type. This conventional active matrix typeliquid crystal display device of digital driving type is composed of, asshown in FIG. 19, such components as a source signal line shift register1401, digital decoder address lines (a-d) 1402, latch circuits (LAT1)1403, other latch circuits (LAT2) 1404, a latch pulse line 1405, D/Aconversion circuits 1406, gradation (gray scale) voltage lines 1407,source signal lines 1408, a gate signal line shift register 1409, gatesignal lines (scanning lines) 1410, and pixel TFTs 1411. Herein, a 4-bitactive matrix type liquid crystal display device of digital driving typeis taken as an example. It should be noted that for simplicity, each ofthe illustrated latch circuits (LAT1 and LAT2) in FIG. 19 actuallyrepresents four of the latch circuits.

Digital signals (digital gradation signals) supplied to the addresslines (a-d) 1402 of the digital decoder are sequentially written ontogroups of the latch circuits LAT1 (LAT1 groups) in accordance withtiming signals supplied from the source signal line shift resister 1401.

A time period required until completion of a set of writing operationsof the digital signals for the LAT1 groups is referred to as “one lineperiod”. More specifically, the one line period corresponds to a timeperiod from the time when the writing operation of the digital signalfrom the digital decoder to the most leftward LAT1 is started until thetime when the writing operation of the digital signal from the digitaldecoder to the most rightward LAT1 is completed.

After the completion of writing operations of the digital signals forthe LAT1 groups, the digital signals thus written into the LAT1 groupsare simultaneously transmitted to and written onto the LAT2 groups atthe time when a latch pulse appears on the latch pulse line inaccordance with the operation timings of the shift register.

Onto the LAT1 groups that have thus transmitted the digital signals tothe LAT2 groups, other digital signals supplied to the digital decoderare again written sequentially in accordance with the signals suppliedfrom the source signal line shift register.

During this second-round one line period, voltages in accordance withthe digital signals transmitted to the LAT2 groups are supplied to thesource signal lines in synchronization with the start of the second-turnone line period. In the exemplary driver circuit described herein,conversion of the digital signals into gradation voltages is implementedby selecting one of the 16 gradation voltage lines by means of the D/Aconversion circuit.

The selected gradation voltage is supplied to the corresponding sourcesignal line during the one line period. The corresponding TFT isswitched in response to a scanning signal from the gate signal lineshift register so that liquid crystal molecules are driven.

By repeating the above-mentioned operations for the number of thescanning lines, one picture (one frame) is created. In general, in theactive matrix type liquid crystal display device, 60 frames of imagesare switched per second.

With reference to FIG. 20, a conventional D/A conversion circuit to beused in the above-mentioned digital driver circuit will be describedbelow.

The conventional 4-bit D/A conversion circuit comprises a plurality ofswitches (sw0-sw15) and a plurality of gradation voltage lines (V0-V15).One of the switches (sw0-sw1S) is selected in accordance with the 4-bitdigital signal supplied from the LAT2 groups, and a voltage is suppliedto the source signal line 1407 from the gradation voltage line coupledto the selected switch.

One D/A conversion circuit as described above is provided substantiallyto each of the source signal lines.

The conventional 4-bit D/A conversion circuit as set forth aboveincludes the 16 switches and the 16 gradation voltage lines.Furthermore, the number of the switches will be required to increase inan exponential manner with an increase in the number of bits. In otherwords, the conventional D/A conversion circuit intended to handle n-bitdigital signal requires 2^(n) switches. Accordingly, in an actual activematrix type liquid crystal display device, a large area is occupied bythe switches, resulting in a large area occupied by the whole drivercircuit. This is disadvantageous for realizing miniaturization of thedevice.

With reference to FIG. 21, another conventional D/A conversion circuitto be used in the above-mentioned digital driver circuit will bedescribed below. In the 4-bit D/A conversion circuit as shown in FIG.21, similarly to the 4-bit D/A conversion circuit as described above,one of the switches (sw0-sw15) is selected in accordance with the 4-bitdigital signal supplied from the LAT2 groups, and a voltage is suppliedto the source signal line from the gradation voltage line coupled to theselected switch.

The D/A conversion circuit shown in FIG. 21 includes five gradationvoltage lines (V0-V4), and the number of the line is less than that ofthe 4-bit D/A conversion circuit in FIG. 20 as described above.

The voltage to be applied across the V0 to V4 is divided by way ofresistors so that voltages of different levels are supplied to therespective five gradation voltage lines (V0-V4). In particular, thehighest voltage is applied to the V4, while the lowest voltage isapplied to the V0.

However, the D/A conversion circuit shown in FIG. 21 has a problem inwhich the overall resistance becomes large because the voltages to beapplied to the respective gradation voltage lines are divided via theresistors, whereby a sufficient writing time of display data to a liquidcrystal panel cannot be obtained.

In addition, when the number of bits increases, a device resistance andwiring resistance also increase.

Furthermore, in order to realize high finer definition of asemiconductor display device, the number of pixels, i.e., the number ofsource signal lines, are required to be increased.

Moreover, as described previously, an increase in the number of sourcesignal lines leads to an increase in the area occupied by the D/Aconversion circuit, as well as increases in the wiring remittances andthe device resistance. These will also prevent high finer definitionfrom being realized.

SUMMARY OF THE INVENTION

In view of the disadvantages as described above, an object of thepresent invention is to provide a D/A conversion circuit capable ofwriting display data to liquid crystal cells with higher precision at ahigher speed, and a semiconductor device utilizing such a D/A conversioncircuit.

According to the present invention, there is provided a D/A conversioncircuit for supplying to an output line a gradation voltagecorresponding to n-bit digital signal (n is a natural number that isequal to or larger than 2) to be input, characterized in that:

-   -   the n-bit digital signal is divided into upper x bit(s) and        lower y bit(s) (where x+y=n; both x and y are natural numbers);    -   adjacent two gradation voltage lines among (2^(x)+1) gradation        voltage lines are selected in accordance with the upper x bit(s)        of the n-bit digital signal;    -   after a first gradation voltage applied to either one of the        selected adjacent two gradation voltage lines is supplied to the        output line, 2^(y) second gradation voltages are generated from        a potential difference between the selected adjacent two        gradation voltage lines by the lower y bit(s) of the n-bit        digital signal; and    -   one of the 2^(y) second gradation voltages is supplied to the        output line.

Further, according to the present invention, there is provided a D/Aconversion circuit for supplying to an output line a gradation voltagecorresponding to n-bit digital signal (n is a natural number that isequal to or larger than 2) to be input, characterized in that:

-   -   the n-bit digital signal is divided into upper x bit(s) and        lower y bit(s) (where x+y=n; both x and y are natural numbers);    -   the z-th and (z+1)-th gradation voltage lines among (2^(x)+1)        gradation voltage lines are selected in accordance with the        upper x bit(s) of the n-bit digital signal (where z is a natural        number in the range from 1 to 2^(x));    -   after a first gradation voltage applied to either one of the        selected z-th and (z+1)-th gradation voltage lines is supplied        to the output line, 2^(y) second gradation voltages are        generated from a potential difference between the selected z-th        and (z+1)-th gradation voltage lines by the lower y bit(s) of        the n-bit digital signal; and    -   one of the 2^(y) second gradation voltages is supplied to the        output line.

Still further, in the above-mentioned respective structures, it ischaracterized in that the D/A conversion circuit is manufactured on aninsulating substrate by using a thin film transistor.

Yet further, in the above-mentioned respective structures, it ischaracterized in that the first gradation voltage is lower than avoltage value applied to the other one of the selected adjacent twogradation voltage lines.

Furthermore, according to the present invention, there is provided asemiconductor device, comprising:

-   -   a plurality of TFTs arranged in matrix; and    -   a source signal line driver circuit and a gate signal line        driver circuit both for driving the plurality of TFTs,    -   characterized in that the source signal line driver circuit        comprises a D/A conversion circuit for supplying to an output        line a gradation voltage corresponding to n-bit digital signal        (n is a natural number that is equal to or larger than 2) to be        input,    -   the n-bit digital signal is divided into upper x bit(s) and        lower y bit(s) (where x+y=n; both x and y are natural numbers);    -   adjacent two gradation voltage lines among (2^(x)+1) gradation        voltage lines are selected in accordance with the upper x bit(s)        of the n-bit digital signal;    -   after a first gradation voltage applied to either one of the        selected adjacent two gradation voltage lines is supplied to the        output line, 2^(y) second gradation voltages are generated from        a potential difference between the selected adjacent two        gradation voltage lines by the lower y bit(s) of the n-bit        digital signal; and    -   one of the 2^(y) second gradation voltages is supplied to the        output line.

Still further, there is provided a semiconductor device, comprising:

-   -   a plurality of TFTs arranged in matrix; and    -   a source signal line driver circuit and a gate signal line        driver circuit both for driving the plurality of TFTs,    -   characterized in that the source signal line driver circuit        comprises a D/A conversion circuit for supplying to an output        line a gradation voltage corresponding to n-bit digital signal        (n is a natural number that is equal to or larger than 2) to be        input,    -   the n-bit digital signal is divided into upper x bit(s) and        lower y bit(s) (where x+y=n; both x and y are natural numbers);    -   the z-th and (z+1)-th gradation voltage lines among (2^(x)+1)        gradation voltage lines are selected in accordance with the        upper x bit(s) of the n-bit digital signal (where z is a natural        number in the range from 1 to 2^(x));    -   after a first gradation voltage applied to either one of the        selected z-th and (z+1)-th gradation voltage lines is supplied        to the output line, 2^(y) second gradation voltages are        generated from a potential difference between the selected z-th        and (z+1)-th gradation voltage lines by the lower y bit(s) of        the n-bit digital signal; and    -   one of the 2^(y) second gradation voltages is supplied to the        output line.

Yet further, according to the present invention, there is provided asemiconductor device, comprising:

-   -   a plurality of TFTs; and    -   a source signal line driver circuit and a gate signal line        driver circuit both for driving the plurality of TFTs,    -   characterized in that the source signal line driver circuit        comprises a D/A conversion circuit for supplying to an output        line a gradation voltage corresponding to n-bit digital signal        (n is a natural number that is equal to or larger than 2) to be        input,    -   the n-bit digital signal is divided into upper x bit(s) and        lower y bit(s) (where x+y=n; both x and y are natural numbers);    -   the z-th and (z+1)-th gradation voltage lines among (2^(x)+1)        gradation voltage lines are selected in accordance with the        upper x bit(s) of the n-bit digital signal (where z is a natural        number in the range from 1 to 2^(x));    -   after a first gradation voltage applied to either one of the        selected z-th and (z+1)-th gradation voltage lines is supplied        to the output line. 2^(y) second gradation voltages are        generated from a potential difference between the selected z-th        and (z+1)-th gradation voltage lines by the lower y bit(s) of        the n-bit digital signal; and    -   one of the 2^(y) second gradation voltages is supplied to the        output line.

Yet still further, in the above-mentioned respective structures, it ischaracterized in that the plurality of TFTs, the source signal linedriver circuit, and the gate signal line driver circuit are integrallymanufactured on an insulating substrate by using a thin film transistor.

Furthermore, Still further, in the above-mentioned respectivestructures, it is characterized in that the first gradation voltage islower than a voltage value applied to the other one of the selectedadjacent two gradation voltage lines.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 shows a schematic diagram illustrating the construction of anactive matrix type liquid crystal display device including a D/Aconversion circuit in accordance with the present invention;

FIG. 2 shows a circuit diagram of a latch circuit;

FIG. 3 shows a diagram illustrating the construction of the D/Aconversion circuit in accordance with the present invention;

FIG. 4 shows a schematic diagram illustrating the construction of anactive matrix type liquid crystal display device including a D/Aconversion circuit in accordance with the present invention;

FIG. 5 shows a diagram illustrating the construction of the D/Aconversion circuit in accordance with the present invention;

FIGS. 6A to 6D show cross-sectional views illustrating manufacturingsteps of a pixel TFT, a holding capacitor, and a TFT in a drivercircuit;

FIGS. 7A to 7D show cross-sectional views illustrating manufacturingsteps of a pixel TFT, a holding capacitor, and a TFT in a drivercircuit;

FIGS. 8A to 8D show cross-sectional views illustrating manufacturingsteps of a pixel TFT, a holding capacitor, and a TFT in a drivercircuit;

FIGS. 9A to 9C show cross-sectional views illustrating manufacturingsteps of a pixel TFT, a holding capacitor, and a TFT in a drivercircuit;

FIG. 10 shows a cross-sectional view of a pixel TFT, a holdingcapacitor, and a TFT in a driver circuit;

FIGS. 11A to 11C show plane views illustrating manufacturing steps of apixel TFT, a holding capacitor, and a TFT in a driver circuit;

FIGS. 12A to 12C show top views illustrating manufacturing steps of apixel TFT, a holding capacitor, and a TFT in a driver circuit;

FIG. 13 shows a cross-sectional view of a liquid crystal display device;

FIGS. 14A to 14F show exemplary diagrams of electronic equipment;

FIGS. 15A to 15D show other exemplary diagrams of electronic equipment;

FIG. 16 shows an exemplary circuit diagram of the D/A conversion circuitin accordance with the present invention;

FIG. 17 shows another exemplary circuit diagram of the D/A conversioncircuit in accordance with the present invention;

FIG. 18 shows still another exemplary circuit diagram of the D/Aconversion circuit in accordance with the present invention;

FIG. 19 shows a diagram illustrating the construction of a conventionaldigital driving type liquid crystal display device;

FIG. 20 shows a D/A conversion circuit to be used in the conventionaldigital driving type liquid crystal display device; and

FIG. 21 shows another D/A conversion circuit to be used in theconventional digital driving type liquid crystal display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, some of embodiments of the present invention will bedescribed.

A D/A conversion circuit in accordance with the present invention allowsa writing operation of a voltage (a true gradation voltage) to beperformed at a higher speed by first applying a first voltage (a voltageclose to the true gradation voltage). which is supplied without passingthrough a resistor element, to an output line and then applying a secondvoltage (the true gradation voltage), which is supplied via the resistorelement, to the output line.

Specifically, in the D/A conversion circuit of the present invention,the first voltage is written to the output line beforehand and then thesecond voltage is written to the output line so as to increase thevoltage level from the first voltage to the true gradation voltage.Thus, a display voltage can be written at a very high speed.

In accordance with the present invention, the D/A conversion circuit forsupplying to an output line a gradation voltage corresponding to n-bitdigital signal (n is a natural number that is equal to or larger than 2)to be input, comprises: a first D/A conversion circuit for dividing then-bit digital signal into upper x bit(s) and lower y bit(s) (wherex+y=n; both x and y are natural numbers) and selecting adjacent twogradation voltage lines among (2^(x)+1) gradation voltage lines inaccordance with the upper x bit(s) of the n-bit digital signal; aplurality of input lines coupled to each other via a plurality ofresistors connected in series with digital signals having differentpotentials being respectively supplied thereto; and a second D/Aconversion circuit. The second D/A conversion circuit includes: a firstswitch circuit designed so that only one of a plurality of switches isclosed in accordance with the applied digital signal; and a secondswitch circuit designed so that a switch is closed immediately beforethe first switch is closed and the thus-closed switch is opened afterapplication of a predetermined voltage value.

The predetermined voltage value may be equal to a voltage value which isbeing applied either one of the adjacent two gradation voltage lines bythe first D/A conversion circuit. In the case where power consumption isrequired to be reduced, among the adjacent two gradation voltage lines,the one to which a lower voltage value is being applied is connected tothe second switch circuit without a resistor element interposedtherebetween.

Furthermore, when the second switch circuit is turned on, only onegradation voltage line (the one to which a lower voltage is beingapplied) may be selected by the first D/A conversion circuit in order toreduce power consumption.

FIG. 3 shows an exemplary configuration of a D/A conversion circuit inaccordance with the present invention. It should be noted that thesecond switch circuit can be designed to also have the function of thefirst switch circuit as shown in FIG. 16. Alternatively, the firstswitch circuit can be designed to also have the function of the secondswitch circuit as shown in FIG. 17. Furthermore, although either one ofthe input lines to which the different voltage values are beingrespectively applied can be connected to the second switch circuit, itis preferable that the input line to which a lower voltage value isbeing supplied is connected to the second switch circuit from theviewpoint of power consumption. In addition, the present invention is ingeneral applicable to a D/A conversion circuit of C2C type.

As can be realized from the above, arrangement and/or connection of theD/A conversion circuit in accordance with the present invention can bearbitrarily modified.

The present invention which has such general construction as describedabove will be described below in more detail with reference to someembodiments thereof.

Embodiment 1

In Embodiment 1, an embodiment of a D/A conversion circuit in accordancewith the present invention will be described. Specifically, the presentembodiment describes details of the D/A conversion circuit provided in asource signal line driver circuit in an active matrix type liquidcrystal display device having pixels of 800 (along a horizontaldirection)×600 (along a vertical direction) for converting a digitalsignal into an analog gradation signal (gradation voltage).

Although the D/A conversion circuit for processing a 4-bit digitalsignal will be described in the present embodiment, the presentinvention is not limited to this case. The present invention can beapplied to any D/A conversion circuits for processing a digital signalof 2 bits or more.

FIG. 1 shows a schematic diagram illustrating the construction of anactive matrix type liquid crystal display device in accordance with thepresent embodiment.

The active matrix type liquid crystal display device in accordance withthe present embodiment is composed of various components such as a firstsource signal line shift register 101, digital decoder address lines (a,b) 102, latch circuits (LAT1.0 to LAT1,799) 103, latch circuits (LAT2,0to LAT2,799) 104, a latch pulse line 105 first D/A conversion circuits(1st-D/A,0 to 1st-D/A,799) 106 gradation voltage lines (V0 to V4) 107,first output lines 108, a second source signal line shift register 109.digital decoder address lines (c, d) 110, latch circuits (LAT3,0 toLAT3,799) 111, latch circuits (LAT4,0 to LAT4,799) 112, a latch pulseline 113, second D/A conversion circuits (2nd-D/A,0 to 2nd-D/A,799) 114,second-output lines 115, a gate signal line shift register 116 as a gatesignal line driver circuit, source signal lines 117, gate signal lines(scanning lines) 118, and pixel TFTs 119.

Although not illustrated in FIG. 1, other components such as a buffer oran analog switch can be appropriately provided.

Among a 4-bit digital signal to be externally supplied, the upper 2 bitsare supplied to the address lines a and b 102, respectively while thelower 2 bits are supplied to the address lines c and d 110,respectively.

Voltages of different levels are supplied to the respective fivegradation voltage lines (V0-V4) 107 by dividing a voltage to be appliedacross the V0 to V4 by wav of resistors. In particular, the highestvoltage is applied to the V4, while the lowest voltage is applied to theV0.

The gradation voltage line to which the lowest voltage is supplied isreferred to as a first gradation voltage line, while the gradationvoltage line to which the highest voltage is supplied is referred to asa fifth gradation voltage line. Thus, among the first through fifthgradation voltage lines, the higher-order gradation voltage lines aresupplied with voltages of higher levels.

The first source line shift resister 101 supplies latch signals (timingsignals) sequentially to the latch circuits LAT1,0 to LAT1,799. Thelatch circuits LAT1.0 to LAT1,799 sequentially receive digital signalsfrom the address lines a and b 102 in accordance with the latch signalssupplied from the first source line shift resister and hold the receiveddigital signals.

At the exact time when the receipt of the digital signal by the latchcircuit LAT1,799 is completed, a latch signal is supplied to the latchpulse line 105, and digital signals are simultaneously received by allof the latch circuits LAT2,0 to LAT2,799 from the latch circuits LAT1,0to LAT1,799, and held therein. The digital signals received by the latchcircuits LAT2,0 to LAT2,799 are transmitted to the first D/A conversioncircuits 106 during the one line period.

FIG. 2 shows a circuit diagram of one of the latch circuits (LAT1,0 andLAT2,0). The latch circuits LAT1,0 and the latch circuits LAT2,0 havethe same configuration as each other.

The latch circuit LAT1,0 includes clocked inverters 201, 203, 204, 206and inverters 202, 205, and receives digital signals from the addresslines a and b 102 to hold them. The latch signal (1at 1,0) from thefirst source signal line shift resister 101 and an inverted signalthereof (inverted 1at1,0) are used for switching the clocked inverters201, 203, 204, 206.

The latch circuit LAT2,0 includes clocked inverters 207, 209, 210, 212and inverters 208, 211, and receives digital signals from the latchcircuit LAT1,0 to hold them. The latch signal (1at2) from the latchpulse line 105 and an inverted signal thereof (inverted 1at2) are usedfor switching the clocked inverters 207, 209, 210, 212. The latchcircuit LAT2,0 transmits the digital signals to the first D/A conversioncircuits.

The digital signals to be supplied to the address lines a and b 102 aresupplied to the first D/A conversion circuits 106 via the thus-explainedtwo-stage latch circuits. Accordingly, for the purpose of explanation,the signal lines connected to the first D/A conversion circuits arereferred to as the a and b signal lines respectively, in the presentembodiment.

The first D/A conversion circuits (1st-D/A,0 to 1st-D/A,799) 106 arerespectively supplied with 2-bit digital signal from the latch circuitsLAT2,0 to LAT2,799. The first D/A conversion circuits (1st-D/A,0 to1st-D/A,799) 106 convert the supplied 2-bit digital signal into analogsignals (gradation voltages), and supply them to the second D/Aconversion circuits (2nd-D/A,0 to 2nd-D/A,799) 114 via the first outputlines 108 (108-1 and 108-2).

In synchronization with the timing when the first source signal lineshift register 101 sequentially transmits the latch signals to the latchcircuits LAT 1,0 to 799, the second source signal line shift register109 sequentially transmits the latch signals to the latch circuitsLAT3,0 to 799. Thus, the first and second source signal line shiftregisters respectively transmit the latch signals to the latch circuitsLAT1,0 and LAT3.0 at the same timings. In addition, the first and secondsource signal line shift registers respectively transmit the latchsignals to the latch circuits LAT1,1 and LAT3,1 at the same timings.

The latch circuits LAT3,0 to LAT3,799 sequentially receive 2-bit digitalsignal from the address lines c and d 110 in accordance with the latchsignals supplied from the second source line shift resister 109, andhold the received digital signals. At the exact time when the receipt ofthe digital signal by the latch circuit LAT3,799 is completed, a latchsignal is supplied to the latch pulse line 113, and digital signals aresimultaneously received by all of the latch circuits LAT4,0 to LAT4,799from the latch circuits LAT3,0 to LAT3,799, and held therein. Thedigital signals received by the latch circuits LAT4,0 to LAT4,799 aretransmitted to the second D/A conversion circuits 114.

The second D/A conversion circuits (2nd-D/A,0 to 2nd-D/A,799) supplygradation voltages to the second output lines 115 connected to thesource signal lines, based on the gradation voltages supplied from theoutput lines 108 of the first D/A conversion circuits and the supplied2-bit digital signal.

The gradation voltages supplied to the second output lines 115 arefurther supplied to the source signal lines 117 via a buffer (notillustrated) or the like. In accordance with scanning signals suppliedfrom the gate signal line shift resister 116, the pixel TFT 119connected to the corresponding gate signal line 118 is turned on so thatthe gradation voltage is applied to liquid crystal molecules.

Thus, all of the pixel TFTs connected to the selected scanning line aresimultaneously turned on to drive liquid crystal molecules. All of thescanning lines are sequentially selected so that an image of 1 frame isgenerated. In the present embodiment, images of 60 frames are generatedper second.

Hereinbelow, the first and second D/A conversion circuits 106 and 114will be described in detail with reference to FIG. 3.

FIG. 3 shows schematic diagrams of the first and second D/A conversioncircuits 106 and 114. Operations of the first and second D/A conversioncircuits 106 and 114 will be first described with reference to FIG. 3.

The first D/A conversion circuit 106 has a switch circuit swA includingfour internal switches (swA1 to swA4), a switch circuit swB includingfour internal switches (swB1 to swB4), and gradation voltage lines 107(V0 to V4). The second D/A conversion circuit 114 has a switch circuitswC including four internal switches (swC1 to swC4), four resistors (R1to R4), and a switch circuit swD. Any inherent resistances of wiringsthemselves are not considered for simplicity.

In the present embodiment, the internal switch swA4 is connected to thegradation voltage line V4, the internal switches swA3 and swB4 areconnected to the gradation voltage line V3, the internal switches swA2and swB3 are connected to the gradation voltage line V2, the internalswitches swA1 and swB2 are connected to the gradation voltage line V1,and the internal switch swB1 is connected to the gradation voltage lineV0.

In the first D/A conversion circuits 106, the 2-bit digital signal to besupplied from the address lines a and b via the latch circuits controlsthe switch circuits swA and swB. In accordance with the digital signalsto be supplied from the address lines a and b via the latch circuitseither one of the four internal switches (swA1 to swA4) in the switchcircuit swA is designed to be closed. Two or more internal switches arenever closed simultaneously. In addition, in accordance with the digitalsignals to be supplied from the address lines a and b, either one of thefour internal switches (swB1 to swB4) in the switch circuit swB isdesigned to be closed. Similarly, two or more internal switches arenever closed simultaneously. Furthermore, closing timings for the fourinternal switches (swA1 to swA4) in the switch circuit swA and those forthe four internal switches (swB1 to swB4) in the switch circuit swBsatisfy the following relationship. The internal switch swB1 is closedwhen the internal switch swA1 is closed, the internal switch swB2 isclosed when the internal switch swA2 is closed the internal switch swB3is closed when the internal switch swA3 is closed, and the internalswitch swB4 is closed when the internal switch swA4 is closed.Accordingly, the adjacent two gradation voltage lines are alwaysselected by the switch circuits swA and swB. Thus, with any input 2-bitdigital signal, the adjacent two gradation voltage lines are selected bythe switch circuits swA and swB and the gradation voltages are suppliedto the first output lines 108 (108-1 and 108-2). Herein, one of thefirst output lines to be selected by the four internal switches of theswitch circuit swA is referred to as the first output line (H) 108-1,while the other first output line to be selected by the four internalswitches of the switch circuit swB is referred to as the first outputline (L) 108-2.

In the second D/A conversion circuits 114, the 2-bit digital signal tobe supplied from the address lines c and d via the latch circuitscontrols the switch circuit swC. In accordance with the digital signalsto be supplied from the address lines c and d via the latch circuits,only one of the four internal switches (swC1 to swC4) in the switchcircuit swC is designed to be closed (turned on).

In addition, when either one of the internal switches (swC1 to swC4) isclosed (turned on), the switch circuit (swD) is designed to be opened(turned off) at least temporarily. When the switch circuit (swD) isclosed, the internal switches (swC1 to swC4) may be closed.

The gradation voltages supplied to the first output line (H) 108-1 andthe first output line (L) 108-2 are applied to the second D/A conversioncircuit 114. In addition the first output line (H) 108-1 and the firstoutput line (L) 108-2 are coupled to each other by the four resistors(R1 to R4) connected in series.

Four different gradation voltages are generated by the four resistors(R1 to R4) of the second D/A conversion circuit based on the gradationvoltages supplied to the first output line (H) 108-1 and the firstoutput line (L) 108-2.

Accordingly, after the switch circuit swD is closed and a voltage closeto the corresponding gradation voltage is supplied to the second outputlines beforehand the switch circuit swD is opened and either one of theinternal switches (swC1 to swC4) of the switch circuit swC is closed,whereby the corresponding gradation voltage is supplied to the secondoutput lines 115. The gradation voltage supplied to the second outputlines 115 is further supplied to the source signal lines 117 via abuffer (not illustrated) or the like.

In the present embodiment, the voltage to be supplied to the firstoutput line (L) 108-2 is supplied to the second output lines in order toreduce power consumption beforehand. However, the present invention isnot limited to such a case. It is possible to design the circuit so thata voltage to be supplied to the first output line (H) 108-1 is suppliedto the second output lines beforehand and the voltage applied via theresistors is subtracted therefrom to thereby obtain the true gradationvoltage.

In the present embodiment, the 4-bit digital signal is divided into theupper 2 bits and the lower 2 bits so that each of these two portions ofthe digital signal is used to control the switching operations of theswitch circuits swA. swB and swC. However, the dividing manner of the4-bit digital signal is not limited to the above case.

For example, upper 3 bits can be used for control switching of theswitch circuits swA and swB, while the least significant 1 bit can beused for control the switching of the switch circuit swC. In this case,the number of internal switches of both the switch circuits swA and swBis 8 (swA1 to swA8, swB1 to swB8), and the number of gradation voltagelines is 9 (V0 to V8). In addition, the number of internal switches ofthe switch circuit swC is 2 (swC1 and swC2), and the number of resistorsis 2 (R1 and P2). After the switch circuit swD is closed and a voltageclose to the corresponding gradation voltage is supplied to the secondoutput lines beforehand, the switch circuit swD is opened and 3-bitdigital signal is input to the switch circuit swA. One of the eightinternal switches of the switch circuit swA is thus closed to select oneof the gradation voltage lines, and the voltage thereof is supplied tothe first output line (H). Similarly, after the switch circuit swD isclosed and a voltage close to the corresponding gradation voltage issupplied to the second output lines beforehand, the switch circuit swDis opened and 3-bit digital signal is input to the switch circuit swB.One of the eight internal switches of the switch circuit swB is thusclosed to select one of the gradation voltage lines, and the voltagethereof is supplied to the first output line (L). Furthermore, 1-bitdigital signal is input to the switch circuit swC. One of the twointernal switches of the switch circuit swC is thus closed so that thecorresponding gradation voltage is supplied to the second output lines.The gradation voltage supplied to the second output lines is furthersupplied to the source signal lines via a buffer or the like.

Although the D/A conversion circuit for handling 4-bit digital signalhas been described in the present embodiment, in accordance with thepresent invention, the D/A conversion circuit for handling n-bit digitalsignal (n is a natural number that is equal to or larger than 2) can berealized. In such a case, the n-bit digital signal can be divided intoupper x bit(s) and lower y bit(s) (where x+y=n). In this case the numberof internal switches of the switch circuit swA is 2^(x)(swA1 to swA2^(x)), the number of internal switches of the switch circuit swB issimilarly 2^(x)(swB1 to swB2 ^(x)), and the number of gradation voltagelines is 2^(x+)1. In addition, the number of internal switches of theswitch circuit swC is 2^(y) (swC1 to swC2 ^(y)), and the number ofresistors is 2^(y) (R1 to R2 ^(y)).

Hereinbelow, a method for manufacturing an active matrix type liquidcrystal display device including a D/A conversion circuit in accordancewith the present embodiment will be described. It should be noted thatthe manufacturing method below is merely one embodiment of the presentinvention, and therefore, the D/A conversion circuit of the presentinvention can be manufactured in accordance with any differentmanufacturing method.

More specifically, a method for manufacturing pixel TFTs in a displayregion and TFTs of a driver circuit to be provided in the vicinity ofthe display region on the identical substrate will be described indetail. For the purpose of simplicity, a CMOS circuit as a fundamentalcircuit of a shift register circuit, a buffer circuit or the like in acontrol circuit, as well as an n-channel TFT for forming a samplingcircuit, will be illustrated.

In FIG. 6A, a low-alkaline glass substrate or a quartz substrate can beused as a substrate 601. In this embodiment, a low-alkaline glasssubstrate is used. In this case, a heat treatment at a temperature lowerby about 10 to 20° C. than the strain point of glass may be performed inadvance. On the surface of this substrate 601 on which TFTs are to beformed, a base film 602 such as a silicon oxide film, a silicon nitridefilm or a silicon oxynitride film is formed in order to prevent thediffusion of impurities from the substrate 601. For example, alamination of a silicon oxynitride film which is fabricated from SiH₄,NH₃, N₂O by, e.g., by plasma CVD into 100 nm thick and a siliconoxynitride film which is similarly -fabricated from SiH₄ and N₂O into200 nm thick are formed.

Next, a semiconductor film 603 a that has an amorphous structure and athickness of 20 to 150 nm (preferably, 30 to 80 nm) is formed by a knownmethod such as plasma CVD or sputtering. In this embodiment, anamorphous silicon film is formed to a thickness of 55 nm by plasma CVD.As semiconductor films which have an amorphous structure, there are anamorphous semiconductor film and a microcrystalline semiconductor film;and a compound semiconductor film with an amorphous structure such as anamorphous silicon germanium film may also be applied. Further, theground film 602 and the amorphous silicon film 603 a can be formed bythe same deposition method, so that the two films can be formed insuccession. By not exposing the base film to the atmospheric air afterthe formation of the base film, the surface of the base film can beprevented from being contaminated, as a result of which the dispersionin characteristics of the fabricated TFTs and the variation in thethreshold voltage thereof can be reduced. (FIG. 6A)

Then, by a known crystallization technique, a crystalline silicon film603 b is formed from the amorphous silicon film 603 a. For example, alaser crystallization method or a thermal crystallization method (solidphase growth method) may be applied, however, here, in accordance withthe technique disclosed in Japanese Patent Laid-Open No. Hei 7-130652,the crystalline silicon film 603 b is formed by the crystallizationmethod using a catalytic element. It is preferred that prior to thecrystallization step, heat treatment is carried out at 400 to 500° C.for about one hour though it depends on the amount of hydrogencontained, so that, after the amount of hydrogen contained is reduced to5 atomic % or less, the crystallization is carried out. The atoms aresubjected to re-configuration to become dense when an amorphous Issilicon film is crystallized; and therefore, the thickness of thecrystalline silicon film fabricated is reduced by about 1 to 15% thanthe initial thickness of the amorphous silicon film (55 nm in thisembodiment). (FIG. 6B)

Then, the crystalline silicon film 603 b is divided into island-shapedportions, whereby island semiconductor layers 604 to 607 are formed.Thereafter, a mask layer 608 of a silicon oxide film is formed to athickness of 50 to 100 nm by plasma CVD or sputtering. (FIG. 6C)

Then, a resist mask 609 is provided, and, into the whole surfaces of theisland semiconductor layers 605 to 607 forming the n-channel type TFTs,boron (B) is added as an impurity element imparting p-type conductivity,at a concentration of about 1×10¹⁶ to 5×10¹⁷ atoms/cm³, for the purposeof controlling the threshold voltage. The addition of boron (B) may beeffected either by the ion doping or it may be added simultaneously whenthe amorphous silicon film is formed. The addition of boron (B) here isnot always necessary, however, the formation of semiconductor layers 610to 612 into which boron is added is preferable for maintaining thethreshold voltage of the n-channel type TFTs within a prescribed range.(FIG. 6D)

In order to form the LDD regions of the n-channel type TFTs in thedriving circuit, an impurity element imparting n-type conductivity isselectively added to the island semiconductor layers 610 and 611. Forthis purpose, resist masks 613 to 616 are formed in advance. As theimpurity element imparting the n-type conductivity, phosphorus (P) orarsenic (As) may be used; here, in order to add phosphorus (P), iondoping using phosphine (PH₃) is applied. The concentration of phosphorus(P) in the impurity regions 617 and 618 thus formed may be set withinthe range of from 2×10¹⁶ to 5×10 ¹⁹ atoms/cm³. In this specification,the concentration of the impurity element contained in the thus formedimpurity regions 617 to 619 imparting n-type conductivity is representedby (n⁻). Further, the impurity region 619 is a semiconductor layer forforming the storage capacitor of the display region; into this region,phosphorus (P) is also added at the same concentration. (FIG. 7A)

Next, the mask layer 608 is removed by hydrofluoric acid or the like,and the step of activating the impurity elements added at the stepsshown in FIG. 6D and FIG. 7A is carried out. The activation can becarried out by performing heat treatment in a nitrogen atmosphere at 500to 600° C. for 1 to 4 hours or by using the laser activation method.Further, both methods may be used together. In this embodiment, thelaser activation method is employed, that is, a linear beam is formedusing a KrF excimer laser beam (with a wavelength of 248 nm); and thenthe entire surface of the substrate on which island semiconductor layersare formed is treated by scanning the beam of the oscillation frequencyat 5 to 50 Hz. the energy density at 100 to 500 mJ/cm². and the overlapratio of the linear beam at 80 to 98%. Any item of the laser irradiationcondition is subjected to no limitation, so that the operator maysuitably select the condition.

Then, a gate insulating film 620 is formed of an insulating filmincluding silicon to a thickness of 10 to 150 nm, by plasma CVD orsputtering. For example a silicon oxynitride film is formed to athickness of 120 nm. As the gate insulating film, another insulatingfilm including silicon may be used as a single layer or a laminatestructure. (FIG. 7B)

Next, in order to form a gate electrode, a first conductive layer isdeposited. This first conductive layer may be comprised of a singlelayer but may also be comprised of a laminate consisting of two or threelayers. In this embodiment, a conductive layer (A) 621 comprising aconductive metal nitride film and a conductive layer (B) 622 comprisinga metal film are laminated. The conductive layer (B) 622 may be formedof an element selected from among tantalum (Ta), titanium (Ti),molybdenum (Mo) and tungsten (W) or an alloy comprised mainly of theabove-mentioned element, or an alloy film (typically, an Mo—W alloy filmor an Mo—Ta alloy film) comprised of a combination of theabove-mentioned elements, while the conductive layer (A) 621 comprisestantalum nitride (TaN), tungsten nitride (WN), titanium nitride (TiN),or molybdenum nitride (MoN). Further, as the substitute materials of theconductive film (A) 621, tungsten silicide, titanium silicide, andmolybdenum silicide may also be applied. The conductive layer (B) maypreferably have its impurity concentration reduced in order to decreasethe resistance thereof, in particular, as for the oxygen concentration,the concentration may be set to 30 ppm or less. For example, tungsten(W) could result in realizing a resistivity of 20 μΩcm or less byrendering the oxygen concentration thereof to 30 ppm or less.

The conductive layer (A) 621 may be set to 10 to 50 nm (preferably, 20to 30 nm), and the conductive layer (B) 622 may be set to 200 to 400 nm(preferably, 250 to 350 nm). In this embodiment, as the conductive layer(A) 621, a tantalum nitride film having a thickness of 30 nm is used,while, as the conductive layer (B) 622, a Ta film having a thickness of350 nm is used, both films being formed by sputtering. In case ofperforming sputtering here, if a suitable amount of Xe or Kr is addedinto the sputtering gas Ar, the internal stress of the film formed isalleviated, whereby the film can be prevented from peeling off. Thoughnot shown, it is effective to form a silicon film, into which phosphorus(P) is doped, to a thickness of about 2 to 20 nm underneath theconductive layer (A) 621. By doing so, the adhesiveness of theconductive film formed thereon can be enhanced, and at the same time,oxidation can be prevented. In addition, the alkali metal elementslightly contained in the conductive film (A) or the conductive film (B)can be prevented from diffusing into the gate insulating film 620. (FIG.7C)

Next, resist masks 623 to 627 are formed, and the conductive layer (A)621 and the conductive layer (B) 622 are etched together to form gateelectrodes 628 to 631 and a capacitor wiring 632. The gate electrodes628 to 631 and the capacitor wiring 632 are formed in such a manner thatthe layers 628 a to 632 a comprised of the conductive layer (A) and thelayers 628 b to 632 b comprised of the conductive layer (B) are formedas one body respectively. In this case, the gate electrodes 629 and 630formed in the driving circuit are formed so as to overlap the portionsof the impurity regions 617 and 618 through the gate insulating film620. (FIG. 7D)

Then, in order to form the source region and the drain region of thep-channel TFT in the driving circuit, the step of adding an impurityelement which imparts p-type conductivity is carried out. Here, by usingthe gate electrode 628 as a mask, impurity regions are formed in aself-alignment manner. In this case, the region in which the n-channelTFT will be formed is covered with resist mask 633 in advance. Impurityregions 634 are formed by ion doping using diborane (B₂H₆). Theconcentration of boron (B) in this region is brought to 3×10²⁰ to 3×10²¹atoms/cm³. In this specification, the concentration of the impurityelement which imparts p-type contained in the impurity regions 634 isrepresented by (p⁺). (FIG. 8A) Next, in the n-channel TFTs, impurityregions that functioned as source regions or drain regions are formed.Resist masks 635 to 637 are formed, an impurity element for impartingthe n-type conductivity is added to form impurity regions 638 to 642.This is carried out by ion doping using phosphine (PH₃), and thephosphorus (P) concentration in these regions is set to 1×10²⁰ to 1×10²¹atoms/cm³. In this specification, the concentration of the impurityelement imparting the n-type contained in the impurity regions 638 to642 formed here is represented by (n⁺). (FIG. 8B) In the impurityregions 638 to 642, the phosphorus (P) or boron (B) which is added atthe preceding steps are contained, however, as compared with thisimpurity element concentration, phosphorus is added here at asufficiently high concentration, so that the influence by the phosphorus(P) or boron (B) added at the preceding steps need not be taken intoconsideration. Further, the concentration of the phosphorus (P) that isadded into the impurity regions 638 is half to one-third of theconcentration of the boron (B) added at the step shown in FIG. 8A; andthus, the p-type conductivity is secured, and no influence is exerted onthe characteristics of the TFTs.

Then, the step of adding an impurity imparting n-type for formation ofthe LDD regions of the n-channel type TFT in the display region iscarried out. Here, by using the gate electrode 631 as a mask, theimpurity element for imparting n-type is added in a self-alignmentmanner. The concentration of phosphorus (P) added is 1×10¹⁶ to 5×10¹⁸atoms/cm³; by thus adding phosphorus at a concentration lower than theconcentrations of the impurity elements added at the steps shown in FIG.7A, FIG. 8A and FIG. 8B, only impurity regions 643 and 644 aresubstantially formed. In this specification, the concentration of theimpurity element for imparting the n conductivity type which impurityelement is contained in these impurity regions 643 and 644 isrepresented by (n⁻). (FIG. 8C)

Thereafter, in order to activate the impurity elements, which are addedat their respective concentrations for imparting n-type or p-typeconductivity, a heat treatment step is carried out. This step can becarried out by furnace annealing, laser annealing or rapid thermalannealing (RTA). Here, the activation step is performed by furnaceannealing. Heat treatment is carried out in a nitrogen atmosphere withan oxygen concentration of 1 ppm or less, preferably 0.1 ppm or less, at400 to 800° C., generally at 500 to 600° C.; in this embodiment, theheat treatment is carried out at 550° C. for 4 hours. Further, in thecase a substrate-such as a quartz substrate which has heat resistance isused as the substrate 601, the heat treatment may be carried out at 800°C. for one hour; in this case, the activation of the impurity elementsand the junctions between the impurity regions into which the impurityelement is added and the channel forming region could be well formed.

By this heat treatment, on the metal films 628 b to 632 b, which formthe gate electrodes 628 to 631 and the capacitor wiring 632, conductivelayers (C) 628 c to 632 c are formed with a thickness of 5 to 80 nm asmeasured from the surface. For example, in the case the conductivelayers (B) 628 b to 632 b are made of tungsten (W), tungsten nitride(WN) is formed; in the case of tantalum (Ta), tantalum nitride (TaN) canbe formed. Further, the conductive layers (C) 628 c to 632 c can besimilarly formed by exposing the gate electrodes 628 to 631 to a plasmaatmosphere containing nitrogen (for example, plasma atmosphere containsnitrogen or ammonia). Further, heat treatment is carried out in anatmosphere containing 3 to 100% of hydrogen at 300 to 450° C. for 1 to12 hours, thus performing the step of hydrogenating the islandsemiconductor layers. It is a step for terminating the dangling bonds ofthe semiconductor layers by the thermally excited hydrogen. As anothermeans for the hydrogenation, plasma hydrogenation (using the hydrogenexcited by plasma) may be performed.

In the case the island semiconductor layers are fabricated by thecrystallization method using a catalytic element from an amorphoussilicon film, a very small amount of the catalytic element remained inthe island semiconductor layers. Of course, it is possible to completethe TFT even in such a state, however, it is more preferable to removethe residual catalytic element at least from the channel forming region.As one of the means for removing this catalytic element, there is themeans utilizing the gettering function of phosphorus (P). Theconcentration of phosphorus (P) necessary to perform gettering is at thesame level as that of the impurity region (n⁺) which is formed at thestep shown in FIG. 8B; by the heat treatment at the activation stepcarried out here, the catalytic element could be gettered from thechannel forming region of the n-channel type and the p-channel typeTFTs. (FIG. 8D)

FIGS. 11A and 12A are top views of the TFT up to the processes at thispoint and the cross sections A-A′ and C-C′ correspond to A-A′ and C-C′of FIG. 8D. In addition, cross sections B-B′ and D-D′ correspond to thecross sectional diagrams of FIGS. 8A and 9A. Top views of FIGS. 11 and12 omit gate insulating films, however, they are formed so as to atleast show gate electrodes 628 to 631 and capacitor wiring 632 formedover island semiconductor layers 604 to 607 in the processes thus far.

After the activation and hydrogenation steps, a second conductive filmis formed as gate wiring. This second conductive film is preferablyformed of a conductive layer (D) comprised mainly of aluminum (Al) orcopper (Cu) that is a low resistance material, and a conductive layer(E) comprised of titanium (Ti), tantalum (Ta), tungsten (W), ormolybdenum (Mo). In this embodiment, the second conductive film isformed by using, as the conductive layer (D) 645, an aluminum (Al) filmcontaining 0.1 to 2 wt % of titanium (Ti) and by using a titanium (Ti)film as the conductive layer (E) 646. The conductive layer (D) 645 maybe formed to a thickness of 200 to 400 nm (preferably, 250 to 350 nm),while the conductive layer (E) 646 may be formed to a thickness of 50 to200 nm (preferably, 100 to 150 nm). (FIG. 9A) Then, in order to formgate wirings connected to the gate electrodes, the conductive layer (E)646 and the conductive layer (D) 645 are etched, whereby gate wirings647 and 648 and a capacitor wiring 649 are formed. The etching treatmentis carried out in such a manner that, at first, by the dry etching usinga mixture gas consisting of SiCl₄, Cl₂ and BCl₃, the portions extendingfrom the surface of the conductive layer (E) to a part of the way of theconductive layer (D) are removed, and, thereafter, the conductive layer(D) is removed by the wet etching using a phosphoric acid etchingsolution, whereby the gate wirings could be formed, maintaining aselective workability with respect to the base film.

FIGS. 11B and 12B show top views of this state, and cross sections A-A′,and C-C′ correspond to A-A′ and C-C′ of FIG. 9B. In FIGS. 11B and 12B,apart of the gate wirings 647 and 648 overlap a part of the gateelectrodes 628, 629 and 631 and they are electrically connected. Suchstate is also clear from cross sectional views of FIGS. 8B and 9B whichcorrespond to cross sections B-B′ and D-D′. The conductive layer (C)which forms the first conductive layer and the conductive layer (D)which forms the second conductive layer are electrically connected.

A first interlayer insulating film 650 is formed of a silicon oxide filmor a silicon oxynitride film having a thickness of 500 to 1500 nm, andcontact holes reaching the source regions or the drain regions, whichare formed in the respective island semiconductor layers, are formed;and source wirings 651 to 654 and drain wirings 655 to 658 are formed.Though not shown, in this embodiment, these electrodes are formed from athree-layer structure which is constituted by continuously forming a Tifilm having a thickness of 100 nm, an aluminum film containing Ti andhaving a thickness of 300 nm, and a Ti film having a thickness of 150 nmby the sputtering method.

Next, as a passivation film 659, a silicon nitride film, a silicon oxidefilm or a silicon oxinitride film is formed to a thickness of 50 to 500nm (typically. 100 to 300 nm). In the case that a hydrogenatingtreatment is carried out in this state. a desirable result is obtainedin respect of the enhancement in characteristics of the TFTs. Forexample, it is preferable to carry out heat treatment in an atmospherecontaining 3 to 100% of hydrogen at 300 to 450° C. for 1 to 12 hours;or, in the case that the plasma hydrogenation method is employed, asimilar effect is obtained. Here, openings may be formed in thepassivation film 659 at the positions at which contact holes forconnecting the pixel electrodes and drain wirings to each other will beformed later. (FIG. 9C)

FIGS. 11C and 12C show top views of this state and cross sections A-A′and C-C′ correspond to A-A′ and C-C′ of FIG. 9C. FIGS. 11C and 12C omitthe first interlayer insulating film, but source wirings 651, 652 and654 and drain wirings 655. 656 and 658 are connected to the source anddrain regions (not shown) formed in the island semiconductor layers 604,605 and 607 through contact holes formed in the first interlayerinsulating film.

Thereafter, a second interlayer insulating film 660 comprising anorganic resin is formed to a thickness of 1.0 to 1.5 μm. As the organicresin, polyimide, acrylic. polyamide, polyimideamide, or BCB(benzocyclobutene), etc., can be used. Here, polyimide of the type that,after applied to the substrate, thermally polymerizes is used it isfired at 300° C., whereby the second interlayer dielectric film isformed. Then a contact hole reaching the drain wiring 658 is formed inthe second interlayer insulating film 660, and pixel electrodes 661 and662 are formed. The pixel electrodes can be formed by using atransparent conductive film in the case a transmission type liquidcrystal panel is to be obtained, while, in the case a reflection typeliquid crystal panel is to be fabricated, the pixel electrodes can beformed by a metal film. In this embodiment, a transmission type liquidcrystal panel is to be fabricated, so that an indium tin oxide (ITO)film is formed to a thickness of 100 nm by the sputtering method. (FIG.10)

In this way, a substrate having the TFTs of the driving circuit and thepixel TFTs of the display region on the same substrate could becompleted. In the driving circuit, there are formed a p-channel TFT 701,a first n-channel TFT 702 and a second n-channel TFT 703, while, in thedisplay region, there are formed a pixel TFT 704 and a storage capacitor705. In this specification, such a substrate is called active matrixsubstrate for convenience.

The p-channel TFT 701 in the driving circuit has a channel formingregion 706, source regions 707 a and 707 b and drain regions 708 a and708 b in the island semiconductor layer 604. The first n-channel TFT 702has a channel forming region 709, an LDD region 710 overlapping the gateelectrode 629 (such an LDD region will hereinafter be referred to asLov), a source region 711 and a drain region 712 in the islandsemiconductor layer 605. The length in the channel direction of this Lovregion is set to 0.5 to 3.0 μm, preferably 1.0 to 1.5 μm. A secondn-channel TFT 703 has a channel forming region 713, LDD regions 714 and715, a source region 716 and a drain region 717 in the islandsemiconductor layer 606. As these LDD regions, there are formed an Lovregion and an LDD region which does not overlap the gate electrode630(such an LDD region will hereafter be referred as Loff); and thelength in the channel direction of this Loff region is 0.3 to 2.0 μm,preferably 0.5 to 1.5 μm. The pixel TFT 704 has channel forming regions718 and 719, Loff regions 720 to 723, and source or drain regions 724 to726 in the island semiconductor layer 607. The length in the channeldirection of the Loff regions is 0.5 to 3.0 μm, preferably 1.5 to 2.5μm. Further, the storage capacitor 705 comprises capacitor wirings 632and 649, an insulating film composed of the same material as the gateinsulating film and a semiconductor layer 727 which is connected to thedrain region 726 of the pixel TFT 704 and in which an impurity elementfor imparting the n conductivity type is added. In FIG. 10, the pixelTFT 704 is of the double gate structure, but may be of the single gatestructure, or may be of a multi-gate structure in which a plurality ofgate electrodes are provided.

Process for forming an active matrix liquid crystal display device shownin FIG. 13 from an active matrix substrate formed through the aboveprocess is next described.

An alignment film 801 is formed on an active matrix substrate of thestate of FIG. 10. An opposing substrate 802 is next prepared. Theopposing substrate comprises a shielding film 803, an opposing electrode804 and an alignment film 805.

Note that a polyimide film is used for the alignment film in thisembodiment so that the liquid crystal molecules orient in parallel withrespect to the substrate. By performing rubbing treatment after formingan alignment film, the liquid crystal molecules are made to orient inparallel having a determined pre-tilt angle.

The active matrix substrate that when through the above processes andthe opposing substrate are stuck together through sealant (not shown)and spacers, etc., by a known cell assembly process. Thereafter liquidcrystal material 806 is injected between the two substrates andcompletely sealed by a sealant. Thus an active matrix liquid crystaldisplay device is completed.

In the present embodiment, the liquid crystal panel is designed tooperate in a TN (Twisted Nematic) mode. Accordingly, a pair ofpolarizing plates are arranged so as to interpose the liquid crystalpanel therebetween in a crossed Nicols condition (in which polarizingaxes of the pair of polarizing plates are disposed perpendicularly toeach other).

Accordingly, it can be understood that the liquid crystal display devicein the present embodiment operates in the so-called normally white modein which the white display is obtained without voltage application.

In the liquid crystal panel in the present embodiment, only one endsurface of the active matrix substrate to which an FPC is attached isexposed outward. The remaining three end surfaces of the active matrixsubstrate are aligned with those of the opposing substrate.

It will be understood that in accordance with the manufacturing methodas described above, the D/A conversion circuit of the present inventioncan be manufactured, on an insulating substrate such as a quartzsubstrate, a glass substrate or the like, integrally with other drivercircuits and other peripheral devices to be included in the activematrix type liquid crystal display device.

As described above the D/A conversion circuit in accordance with thepresent invention can exhibit various features, and can provide a brightand fine image through the combined effects of these features, therebyrealizing an electro-optical device having satisfactory operatingcharacteristics and high reliability. Furthermore, an electronicequipment of high performances can be obtained in which such anelectro-optical device is mounted as a component.

Furthermore, although the present embodiment has been described bytaking a liquid crystal display device as an example, the presentinvention is applicable to other active matrix type display device, suchas an EL (Electro-Luminescence) display device, and an EC(Electro-Chromics) display device.

Embodiment 2

In Embodiment 2, another configuration of a D/A conversion circuit inaccordance with the present invention will be described. Although an8-bit D/A conversion circuit will be described in the presentembodiment, the present invention is not limited to this case. Thepresent invention can be applied to any D/A conversion circuits forhandling a digital signal of 2 bits or more.

The present embodiment describes the D/A conversion circuit to beprovided in a driver circuit of a liquid crystal display device havingpixels of 1920 (along a horizontal direction)×1080 (along a verticaldirection).

With reference to FIG. 4, FIG. 4 shows a schematic diagram illustratingthe construction of a liquid crystal display device in accordance withthe present embodiment. The liquid crystal display device in accordancewith the present embodiment is composed of various components such as afirst source signal line shift register 501, digital decoder addresslines (a, b, c, d) 502, latch circuits (LAT1,0 to LAT1,1919) 503, latchcircuits (LAT2,0 to LAT2,1919) 504, a latch pulse line 505 switchingcircuits 506, first D/A conversion circuits (1st-D/A,0 to 1st-D/A,479)507, gradation voltage lines (V0 to V16) 508, first output lines 509(509-1 and 509-2) of the first D/A conversion circuits, a second sourcesignal line shift register 510, digital decoder address lines (e, f, g,h) 511, latch circuits (LAT3,0 to LAT3,1919) 512, latch circuits (LAT4,0to LAT4,1919) 513, a latch pulse line 514, switching circuits 515 secondD/A conversion circuits (2nd-D/A,0 to 2nd-D/A,479) 516, second outputlines 517 of the second D/A conversion circuits, switching circuits 518,a gate signal line shift register 519, source signal lines 520, gatesignal lines (scanning lines) 521, and pixel TFTs 522.

Among a 8-bit digital signal to be externally supplied, the upper 4 bitsare supplied to the address lines a, b, c and d, respectively, while thelower 4 bits are supplied to the address lines e, f, g and h,respectively.

Voltages of different levels are supplied to the respective seventeengradation voltage lines (V0-V16) 508 by dividing a voltage to be appliedacross the V0 to V16 by way of resistors. In particular, the highervoltage is applied to the V16, as compared to V0. Thus, similar as inEmbodiment 1, voltage levels to be applied increase from the V0 towardsthe V16.

The first source signal line shift resister 501 supplies latch signalssequentially to the latch circuits 503 (LAT1,0 to LAT1,1919). The latchcircuits 503 receive digital signals from the address lines 502 (a, b,c, d) at the timings when the latch signals are to be input, and holdthe received digital signals. Furthermore, latch signals are supplied tothe latch circuits 504 (LAT2,0 to LAT2,1919), and the digital signalsare provided to the latch circuits 504 from the latch circuits 503 andheld therein. These steps are similar to the corresponding steps inEmbodiment 1, and therefore, the descriptions thereof are omitted here.

The 4-bit digital signal received by and held in the latch circuits 504(LAT2.0 to LAT2,1919) is input to the switching circuits 506. In thepresent embodiment the respective first and second D/A conversioncircuits 501 and 510 are provided for every 4 source signal lines.Accordingly, selection of the latch circuits has to be conducted by theswitching circuits 506. In an actual operation, each of the latchcircuits is selected for a quarter of the one line period. Details ofoperations of the switching circuits 506 are described in Example 1 ofJapanese Patent Application No. Hei 9-286098 filed by the sameapplicant.

Since a pair of the D/A conversion circuits (the first and second D/Aconversion circuits) is provided for every four source signal lines eachof the four latch circuits LAT2,0 to 3 is selected by the switchingcircuits 506 for a quarter of the one line period to supply a 4-bitdigital signal to the first D/A conversion circuits (1st-D/A,0).

The 4-bit digital signal is converted into the gradation voltages by thefirst D/A conversion circuits 507 and supplied to the second D/Aconversion circuits 516.

The second source signal line shift resister 510 supplies latch signalssequentially to the latch circuits 512 (LAT3,0 to LAT3,1919). The latchcircuits 512 receive digital signals from the address lines 511 (e, f,g, h) at the timings when the latch signals are input, and hold thereceived digital signals. Furthermore, latch signals are supplied to thelatch circuits 513 (LAT4,0 to LAT4,1919), and the digital signals areprovided to the latch circuits 513 from the latch circuits 512 and heldtherein. These steps are similar to the corresponding steps inEmbodiment 1, and therefore, the descriptions thereof are omitted here.The first and second source signal line shift registers respectivelytransmit the latch signals to the latch circuits 503 (LAT1.0 toLAT1,1919) and the latch circuits 512 (LAT3,0 to LAT3,1919) at the sametimings.

The 4-bit digital signal received by and held in the latch circuits(LAT4.0 to LAT4,1919) is input to the switching circuits 515. Selectionof the latch circuits is again required to be conducted by the switchingcircuits 506. Each of the latch circuits is selected for a quarter ofthe one line period. Thus, the 4-bit digital signal is sequentiallyreceived by the second D/A conversion circuits 516 from the latchcircuits.

The second D/A conversion circuits 516 supply gradation voltages to theoutput lines 517, based on the digital signals to be input.

Hereinbelow, the first and second D/A conversion circuits in the presentembodiment will be described with reference to FIG. 5. FIG. 5 showsschematic diagrams of the first and second D/A conversion circuits 507and 516. Operations of the first and second D/A conversion circuits 507and 516 will be first described with reference to FIG. 5.

The first D/A conversion circuit 507 has a switch circuit swA including16 internal switches (swA 1 to swA 16), a switch circuit swB including16 internal switches (swB1 to swB16), and 17 gradation voltage lines (V0to V16). The second D/A conversion circuit 516 has a switch circuit swCincluding 16 internal switches (swC1 to swC16), 16 resistors (R1 toR16), and a switch circuit swD. A resistance value of each of theresistors (R1 to R16) can be designed to be large enough to prevent thedevice from being damaged by application of a large current between thefirst output line (H) 509-1 and the first output line (L) 509-2. Suchlarge resistance values will be also advantageous for reducing powerconsumption. The switch circuit swD is not limited to a specific circuitconfiguration, so long as it has a switching function. Any inherentresistances of wirings themselves are not considered for simplicity.

In the first D/A conversion circuits 507, the 4-bit digital signal to besupplied from the address lines a, b, c and d via the latch circuitsselected by the switching circuits 506 controls the switch circuits swAand swB. In accordance with the digital gradation signals to be suppliedfrom the address lines a, b, c, and d via the latch circuits, either oneof the 16 internal switches (swA1 to swA16) in the switch circuit swA isdesigned to be closed. Two or more internal switches are never closedsimultaneously. In addition in accordance with the digital signalssupplied from the address lines a b, c and d via the latch circuits,either one of the 16 internal switches (swB1 to swB16) in the switchcircuit swB is designed to be closed. Similarly, two or more internalswitches are never closed simultaneously. Furthermore, closing timingsfor the 4 internal switches (swA1 to swA16) in the switch circuit swAand those of the 4 internal switches (swB1 to swB16) in the switchcircuit swB satisfy the following relationship. The internal switch swB1is closed when the internal switch swA1 is closed, the internal switchswB2 is closed when the internal switch swA2 is closed, the internalswitch swB3 is closed when the internal switch swA3 is closed and theinternal switch swB4 is closed when the internal switch swA4 is closed.More generally speaking, the internal switches swAn and swBn (where n isa natural number in the range of from 1 to 16) are designed to besimultaneously closed. Accordingly, the adjacent two gradation voltagelines are always selected by the switch circuits swA and swB. Thus, theadjacent two gradation voltage lines are selected by the switch circuitsswA and swB, and the gradation voltages are supplied to the first outputline (H) 509-1 and the first output line (L) 509-2.

In the second D/A conversion circuits 516, the 4-bit digital signal tobe supplied from the address lines e, f, g and h via the latch circuitscontrols the switch circuit swC. In accordance with the digital signalsto be supplied from the address lines e, f, g and h via the latchcircuits, only one of the 16 internal switches (swC1 to swC16) in theswitch circuit swC is designed to be closed.

It should be noted, however, that before the switch circuit swC isclosed the switch circuit swD is closed so that a voltage close to thecorresponding gradation voltage is supplied to the source signal linesbeforehand. In addition, when the switch circuit swC is closed, theswitch circuit swD is designed to be opened at least temporarily.Although not illustrated, a different signal line is provided to which asignal for controlling the turning on/off of the switch circuit swD isapplied.

Sixteen different gradation voltages are generated by the 16 resistors(R1 to R16) based on the gradation voltages supplied to the first outputline (H) 509-1 and the first output line (L) 509-2. After the switchcircuit swD is closed and a voltage close to the corresponding gradationvoltage is supplied to the second output lines beforehand, the switchcircuit swD is opened and either one of the 16 internal switches of theswitch circuit swC is closed, whereby the corresponding gradationvoltage is supplied to the second output lines 517. The gradationvoltage supplied to the second output lines 517 is further supplied tothe source signal lines via a buffer (not illustrated) or the like.

In the present embodiment, the upper 4 bits of the 8-bit digital signalare used to select one of the 16 different gradation voltages, while thelower 4 bits of the same 8-bit digital signal are used to select one ofthe further 16 different gradation voltages. Accordingly, the total of256 different gradation voltages (=16(in connection with the upper 4bits)×16(in connection with the lower 4 bits)) can be selected.

Embodiment 3

In Embodiment 3, an example of a D/A conversion circuit that isdifferent from that in Embodiment 1 will be described with reference toFIG. 16. Only a portion of the switch circuits in the circuitconfiguration in FIG. 16 is different from that in FIG. 3. The samecomponents bear the same reference numerals in these two drawings.

In the circuit configuration shown in FIG. 16, the switch circuit swDcan also have a function as one of the internal switches (swC1 to swC4)of the switch circuit swC. In the present embodiment, the switch circuitswD (swC1) shown in FIG. 16 is designed to be turned on/off inaccordance with a signal for controlling a timing of the preparatoryapplication of the voltage value close to the true gradation voltage anda signal for controlling timings of the turn-on/off operations of theinternal switch swC1. Thus, the number of switching elements to berequired can be reduced, thereby resulting in miniaturization of theoverall circuit size.

Embodiment 4

In Embodiment 4, an example of a D/A conversion circuit that isdifferent from that in Embodiment 1 will be described with reference toFIG. 17. Only a portion of the switch circuits in the circuitconfiguration in FIG. 17 is different from that in FIG. 3. The samecomponents bear the same reference numerals in these two drawings.

In the circuit configuration shown in FIG. 17, one of the internalswitches (swC1 to swC4) of the switch circuit swC can also has afunction as the switch circuit swD. In the present embodiment, theswitch circuit swC1 (swD) shown in FIG. 17 is designed to be turnedon/off in accordance with a signal for controlling timings of theturn-on/off operations of the internal switch swC1 and a signal forcontrolling a timing of the preparatory application of the voltage valueclose to the true gradation voltage. Thus, the number of switchingelements to be required can be reduced, thereby resulting inminiaturization of the overall circuit size.

Embodiment 5

In Embodiment 5, an example of a D/A conversion circuit that isdifferent from that in Embodiment 1 will be described with reference toFIG. 18. The circuit configuration in FIG. 18 is the same as that shownin FIG. 3, except that a buffer circuit is additionally provided. Thesame components bear the same reference numerals in these two drawings.

In the circuit configuration shown in FIG. 18, the second D/A conversioncircuit 1114 is provided with a buffer circuit 1113 (which is anoperational amplifier, a source follower, or the like) connected to thesecond output line 1115. Such a circuit configuration is preferablesince an input to the buffer circuit can be of high impedance with thiscircuit configuration, which can in turn allow requirements for theanalog switch to be mitigated.

Embodiment 6

The present invention is applicable to the overall range of theconventional IC technology. More specifically, the present invention canbe applied to any kinds of semiconductor circuits commercially availablein the current market. For example, the present invention can be appliedto microprocessors integrated in one chip, such as a R1SCmicroprocessor, an ASIC microprocessor, or the like. The presentinvention is also applicable to various circuits ranging from a signalprocessing circuit to be used in a DIA converter or the like to a highfrequency circuit to be used in a portable equipment (such as a mobilephone, a PHS, or a mobile computer).

Embodiment 7

The circuits manufactured by implementing the present invention can beutilized in various electro-optical devices. Namely, the presentinvention can be implemented on all of the electronic appliances thatincorporate such electro-optical devices as display medium.

Examples of such electronic appliances include a video camera, a digitalcamera, a head mounted display (a goggle type display), a wearabledisplay, a car navigation system, a personal computer and a portableinformation terminal (a mobile computer, a cellular telephone, anelectronic book, etc.). FIG. 14A to 14F show examples of these.

FIG. 14A shows a personal computer, which comprises: a main body 2001;an image input section 2002; a display region 2003; and a keyboard 2004.The present invention can be applied to the display region 2003 or otherdriver circuits.

FIG. 14B shows a video camera, which comprises: a main body 2101; adisplay device 2102; a sound input section 2103; an operation switch2104; a battery 2105; and an image receiving section 2106. The presentinvention can be applied to the display device 2102, the sound inputsection 2103 or other signal control circuits.

FIG. 14C shows a mobile computer, which comprises: a main body 2201; acamera section 2202; an image receiving section 2203; an operationswitch 2204; and a display device 2205. The present invention can beapplied to the display region 2205 or other driver circuits.

FIG. 14D shows a goggle type display, which comprises: a main body 2301;a display device 2302; and an arm section 2303. The present inventioncan be applied to the display region 2302 or other driver circuits.

FIG. 14E shows a player that uses a recording medium storing a program(hereinafter called the “recording medium”). It comprises a main body2401, a display region 2402, a speaker unit 2403, a recording medium2404 and an operation switch 2405. Note that by using DVD (digitalversatile disc), CD, etc., as a recording medium of this device, musicappreciation, film appreciation games or the use for Internet can beperformed. The present invention can be applied to the display region2402 or other driver circuits.

FIG. 14F shows a digital camera, which comprises: a main body 2501; adisplay region 2502; a view finder section 2503; an operation switch2504; and an image receiving section (not shown). The present inventioncan be applied to the display region 2502 or other driver circuits.

As described above, the applicable range of the present invention isvery large and it can be applied to electronic appliances of variousfields. The electronic appliances of the present Embodiment can berealized by using constitutions of any combination of Embodiments 1 to6.

Embodiment 8

The TFTs manufactured by implementing the present invention can beutilized invarious electro-optical devices. Namely, the presentinvention can be implemented on all of the electronic appliances thatincorporate such electro-optical devices as display medium.

Projectors (front type or rear type) or the like can be given as suchelectronic appliances. An example is shown in FIGS. 15A to 15D.

FIG. 15A is a front type projector, which comprises a projection system2601 and a screen 2602. The present invention can be applied to theprojection system or other driver circuits.

FIG. 15B is a rear type projector, which comprises a main body 2701 aprojection system 2702, a mirror 2703 and a screen 2704. The presentinvention can be applied to the projection system or other drivercircuits.

FIG. 15C is a diagram showing an example of the structure of theprojection system 2601 and 2702 in FIGS. 15A and 15B. Projectionssystems 2601 and 2702 comprise: an optical light source system 2801;mirrors 2802 and 2804 to 2806: a dichroic mirror 2803; a prism 2807 aliquid crystal display device 2808; a phase differentiating plate 2809;and a projection optical system 2810. The projection optical system 2810comprises an optical system including a projection lens. Though thepresent Embodiment shows an example of three plate type, it is notspecifically limited to this type, and it may for instance be a singleplate type. Further, the operator may appropriately dispose in theoptical path shown by an arrow in FIG. 15C, an optical lens, a filmhaving light polarizing function, a film for adjusting phase difference,or an IR film, etc.

FIG. 15D is a diagram showing an example of the structure of the opticallight source system 2801 of FIG. 15C. In this Embodiment the opticallight source system 2801 comprises a reflector 2811, light sources 2812,2813 and 2814 light polarizer conversion element 2815 and a condenserlens 2816. Note that the optical light source system shown in FIG. 15Dis merely an example and the structure is not specifically limited tothis example. For example, the operator may appropriately dispose anoptical system such as an optical lens, a film having light polarizingfunction, a film for adjusting phase difference, IR film, etc., in theoptical light source system.

As described above, the applicable range of the present invention isvery large, and it is possible to apply the invention to electronicappliances of various areas. Further, the electronic appliances of thisEmbodiment can be realized by using constitutions of any combination ofEmbodiments 1 to 5.

As described above, a D/A conversion circuit in accordance with thepresent invention allows a writing operation of a voltage (a truegradation voltage) to be performed at a higher speed by first applying afirst voltage (a voltage close to the true gradation voltage), which issupplied without passing through a resistor element, to an output lineand then applying a second voltage (the true gradation voltage), whichis supplied via the resistor element, to the output line.

Specifically, in the D/A conversion circuit of the present invention,the first voltage is written to the output line beforehand and then thesecond voltage is written to the output line so as to increase thevoltage level from the first voltage to the true gradation voltage.Thus, a display voltage can be written at a very fast speed, as comparedto the conventional art.

1. A D/A conversion circuit for supplying to an output line a gradationvoltage corresponding to n-bit digital signal (n is a natural numberthat is equal to or larger than 2) to be input, comprising: a first D/Aconversion circuit having a plurality of gradation voltage lines, afirst switching circuit, and second switching circuit; and a second D/Aconversion circuit having at least a plurality of resistors and a thirdswitching circuit, wherein the n-bit digital signal is divided intoupper x bit(s) and lower y bit(s) (where x+y=n; both x and y are naturalnumbers), wherein adjacent two gradation voltage lines among (2^(x)+1)gradation voltage lines are selected in accordance with the upper xbit(s) of the n-bit digital signal in said first D/A conversion circuit,wherein after a first gradation voltage applied to either one of theselected adjacent two gradation voltage lines is supplied to the outputline, 2^(y) second gradation voltages are generated from a potentialdifference between the selected adjacent two gradation voltage lines bythe lower y bit(s) of the n-bit digital signal in said second D/Aconversion circuit, and wherein one of the 2^(y) second gradationvoltages is supplied to the output line.
 2. A circuit according to claim1, wherein the D/A conversion circuit is manufactured on an insulatingsubstrate by using a plurality of thin film transistors.
 3. A circuitaccording to claim 1, wherein the first gradation voltage is lower thana voltage value applied to the other one of the selected adjacent twogradation voltage lines.
 4. A D/A conversion circuit for supplying to anoutput line a gradation voltage corresponding to n-bit digital signal (nis a natural number that is equal to or larger than 2) to be input,comprising: a first D/A conversion circuit having a plurality ofgradation voltage lines, a first switching circuit, and second switchingcircuit; and a second D/A conversion circuit having at least a pluralityof resistors and a third switching circuit, wherein the n-bit digitalsignal is divided into upper x bit(s) and lower y bit(s) (where x+y=n;both x and y are natural numbers), wherein the z-th and (z+1)-thgradation voltage lines among (2^(x)+1) gradation voltage lines areselected in accordance with the upper x bit(s) of the n-bit digitalsignal (where z is a natural number in the range from 1 to 2^(x)) in thefirst D/A conversion circuit, wherein after a first gradation voltageapplied to either one of the selected z-th and (z+1)-th gradationvoltage lines is supplied to the output line, 2^(y) second gradationvoltages are generated from a potential difference between the selectedz-th and (z+1 )-th gradation voltage lines by the lower y bit(s) of then-bit digital signal in the second D/A conversion circuit, and whereinone of the 2^(y) second gradation voltages is supplied to the outputline.
 5. A circuit according to claim 4, wherein the D/A conversioncircuit is manufactured on an insulating substrate by using a pluralityof thin film transistors. 6-15 (Canceled).